A. M. Jordan

Recent developments in the ecology and methods of control of tsetse flies ( Glossina spp.) (Dipt., Glossinidae)—a review

The literature published during 1964–72 concerned with the ecology and control of Glossina is reviewed and put into context with previously published information. Thirty species or subspecies of Glossina are recognised. Improved methods of sampling have been developed which catch many more females of the morsitans group than other methods and allow more accurate estimates of population density and structure to be made than has been possible hitherto. New concepts of the activity and behaviour patterns of G. morsitans Westwood have been developed from work carried out both in the field and in the laboratory. Detailed studies on resting sites have been followed by successful control campaigns based on the selective use of insecticides. Most large-scale control campaigns during the period under review were based on applications of insecticides from the ground; other control methods used were aerial applications of insecticides, mechanical clearing of vegetation, selective game destruction and a small-scale project using the sterile insect release method. Techniques for identifying and assessing trypanosome infections in Glossina are described and a Table is given of infection rates in various species of tsetse. Differences in observed infection rates can result according to the time and method of sampling but actual infection rates are affected by a variety of factors including the maintenance temperature of puparia and adults, the age of the fly at the time of the infective feed, the age structure of the tsetse population and the hosts on which it feeds. Genetic differences in infectibility may occur between different individuals of a single species and certainly occur between different species.

The hosts of Glossina as the main factor affecting trypanosome infection rates of tsetse flies in Nigeria

An examination is made of previously published data on the trypanosome infection rate and the natural hosts, as determined by serological tests, of 9 species of Glossina from various localities in Nigeria. A relationship is demonstrated between a high total infection rate and a large proportion of meals derived from Bovidae (particularly bushbuck (Tragelaphus scriptus), with buffalo (Syncerus nanus) important in some localities). Infection rates and the natural hosts, as determined by the measurement of red blood cells found in the flies, of two species of Glossina in Northern Nigeria were recorded by Lloyd, Johnson, Young and Morrison (1924). An examination of their data shows that a similar relationship exists between feeds derived from Bovidae and total trypanosome infection rate. Species of Glossina which had fed mainly on Suidae usually had fewer infections of T. vivax group of trypanosomes than those which had fed mainly on Bovidae; the numbers of T. congolense group infections were approximately the same in the two groups of species. It is suggested that, at least in Nigeria, the source of food of the flies is the main factor involved in determining the level of infection with trypanosomes in Glossina. The importance of other factors, particularly temperature, is discussed.

Survival and behaviour of tsetse flies (Diptera, Glossinidae) released in the field: a comparison between wild flies and animal-fed and in vitro-fed laboratory-reared flies

Male and female Glossina morsitans morsitans Westw. which emerged from puparia produced by animal-fed and in vitro-fed colonies in England were marked distinctively with non-toxic paint and released into a natural habitat of G. morsitans and G. pallidipes Aust. in Rhodesia. Concurrently, adults of both species which emerged from locally-collected puparia were marked and released. Recaptures from artificial refuges, odour attractants and mobile baits at periods up to 59 days after release and at distances up to 1800 m from the release site indicated no clear differences between native G. morsitans and the two laboratory-reared groups in respect of body size, amount of fat present at emergence, survival, dispersal, availability to a range of baits, diet, speed of taking a first meal, wing damage and insemination rate. Although the blood-meal identifications for marked female G. morsitans were similar to those for both sexes of unmarked flies, blood-meals from marked males showed a relatively high proportion of bovid identifications. Unmarked flies caught were generally older than marked catches. The ratio of females to males in unmarked samples (1:1 for G. morsitans , 2:1 for G. pallidipes ) was roughly double that in marked catches.

A promising method for rearing Glossina austeni (newst.) on a small scale, based on the use of rabbits' ears for feeding

Abstract By manually applying cages to the ears of ordinary rabbits and coaxing the tsetse flies to feed, we have for the first time approached the full reproductive potential of G. austeni and reared an insect which does not seem to be physically inferior to that found in nature. The tedious coaxing process has been overcome by strapping cages to the ears of lop-eared rabbits, but at a small loss in pupal weight. Our preliminary findings suggest that a method has been found which should enable a research worker to keep his own small colony of tsetse and achieve an excellent survival rate for trypanosome transmission experiments.

Tsetse-flies (Glossinidae)

The Glossinidae, or tsetse-flies, form a monogeneric family of the Diptera. The adults range in length from 6 to 14 mm and in all the 23 known species are various shades of brown — ranging from light yellowish brown to dark blackish brown. In some species the abdomen has alternate darker and lighter bands. Female flies give birth, at intervals of about nine days, to a single third-instar larva which rapidly burrows into the soil and transforms into a black puparium; according to the species, this varies in length from 3 to 8 mm.

A method of maintaining Glossina austeni newst. singly, and a study of the feeding habits of the female in relation to larviposition and pupal weight.

To permit detailed study of individual tsetse, either as insects or as the vectors of trypanosomiasis, a method has been devised which resulted in excellent survival and reproduction when tested on females or Glossina austeni Newst. Each mated female was maintained in asmall cage comprising a stainless steel frame covered with non-absorbent black Terylene netting through which the larvae and faeces fell into a tray. Each cage was applied daily to the ear of a rabbit, and records were kept of the number of feeds, time taken to feed, numbers of larvae produced, lengths of interlarvel periods and weights of pupae. The following conclusions are tentative as they are based on detailed studies of only eight females. Five of these produced larvae regularly at 8½- to 9-day intervals. Ninety-six interlarval periods were investigated. Although the mean interval between meals was two days, the flies tended to fast before larviposition and to engorge repeatedly afterwards. Among nearly regular breeders, failure to larviposit did not seem to interrupt the normal feeding rhythum: the reproductive rhythum later re-integrated with the feeding ehythum. A tendency to deviate from the normal feeding pattern either by too frequent feeding after larviposition or by prologation of the fasting period before larviposition, may possible be related to pemature death, but flies which tend to feed on the day before larviposition centainly risk death from pupation in utero —a condition found in 5 per cent. of dead rabbit-fed flies; the complex of pressures which lead to this condition is described. Limited data suggest that females which live to a great age enter a post-productive period when about 6½ month old; the feeding pattern breaks down completely, the fly fasting for periods of up to 11 days, or even longer in the immediate pre-death period. The maximum output of reasonably sized pupae is probably about 20 for G. austeni . Pupal weight increased until the parent flies were about 90 days old, but there was much individual variation; thereafter the weight remained high, but among very old dlies it fell towards the end of reproductive life. Certain individuals tended to produce heavy pupae, a characteristic probable associated with the size of the fly and the total weight of blood taken up in each interlarval period—not with the frequency of feeding; a very infrequent feeder may, however, produce underweight pupae at irregular intervals.

Tsetse eradication plans for southern Africa

Abstract African trypanosomiasis has long been recognized as a serious barrier to cattle production in Africa. The tsetse fly vectors and the three main agents of the disease, Trypanosoma congolense , T. vivax, and T. brucei, exclude domestic livestock from a vast area. The disease is often fatal to cattle and also causes severe losses in livestock production due to poorgrowth, weight loss, low milk yield, infertility, abortion, and reduced capacity for work by draught animals. But this year, an ambitious plan is being launched to study the feasibility of eliminating tsetse flies from a major fly belt in Malawi, Mozambique, Zambia and Zimbabwe. The proposals have attracted heated criticism from some sources, especially those concerned with the environmental impact of largescale insecticide application and the risk of uncontrolled overgrazing by cattle in areas reclaimed from the tsetse flies. In this report, Tony Jordan takes a cool look at the plans

The inseminating potential of male Glossina austeni Newst. and G. morsitans morsitans Westw. (Dipt., Glossinidae)

In laboratory experiments individual male Glossina austeni Newst. were each mated, at two- or three-day intervals, to nine different females. In one series of matings the males were restricted to one pairing and in a second series the sexes were left together for 24 h, when more than one copulation could have occurred. The technique followed in the second series was repeated with G. morsitans morsitans Westw. when the males were presented successively with 12 different females. The fecundity of female G. austeni , after mating with males previously mated on 0 to 5 occasions, was consistently higher when left with their mates for 24 h (mean puparia produced per mature female day=0·102) than when restricted to a single copulation (0.085). When the sexes were left together for 24 h, female fecundity of both species was maximal when mated with males used for up to six times ( ca 0·10 puparia produced per mature female day) but became progressively poorer as the males were used more frequently (0·022 for G. m. morsitans females mated with males used for the twelfth time). The testes of muchmated males are markedly smaller than those of virgin males of the same age. Maximum disposable output of males from colonies of Glossina engaged in production for the sterile insect release method could be obtained by using males retained in the colony for mating on six occasions. It is speculated that in nature the absence of both teneral and old males from the following swarm ensures the presence of only fully sexually potent males in the swarm.

Extracellular ducts within the wall of the spermatheca of tsetse flies ( Glossina spp.) (Dipt., Glossrnidae)

Each spherical spermatheca of Glossina austeni Newst. is composed of a cuticular lining of the lumen which is surrounded by a layer of secretory cells. When the spermatheca is immersed in Amanns lactophenol the cells of the cellular layer lift away from the cuticular intima revealing numerous clubbed processes projecting outwards from the surface of the intima. Sections of spermathecae examined by transmission electron microscopy show that one clubbed process is associated with each cell of the cellular layer. The “porous” head of the process is within a fluidfilled cavity in the cell, lined with microvilli. The stalk of each process is an extracellular duct down which secretions from the cellular layer pass into the lumen of the spermatheca. It is considered that these secretions are the medium, probably containing nutrients, in which spermatozoa are held while in the spermatheca.

Pupal weight in relation to female age in Glossina austeni Newst.

All of the pupae, totalling over 3,750, produced by 320 laboratory-bred females of Glossina austeni Newst. allowed to feed from rabbits’ ears, were weighed soon after deposition. Pupal weights were relatively low initially but increased during the first 30 days of the females’ productive life, were high during the next 100 days or so and thereafter declined to less than the initial level.